A great many methods of encoding and decoding audio signals have been developed up to now. Particularly, in these days, IS13818-7 which is internationally standardized in ISO/IEC is publicly known and highly appreciated as an encoding method for reproducing high quality sound with high efficiency. This encoding method is called AAC. In recent years, the AAC has been adopted to the standard called MPEG-4, and a system called MPEG-4 AAC that has some extended functions added to the IS13818-7 has been developed. An example of the encoding procedure is described in the informative part of the MPEG-4 AAC.
Following is an explanation for an audio encoding device using the conventional encoding method referring to FIG. 1. FIG. 1 is a block diagram that shows the structure of a conventional encoding device 300. The encoding device 300 includes a spectrum amplifying unit 301, a spectrum quantizing unit 302, a Huffman coding unit 303 and an encoded data stream transfer unit 304. A discrete audio signal stream on the time axis obtained by sampling an analog audio signal at a predetermined frequency is divided into every predetermined number of samples at a predetermined time interval, transformed into data on the frequency axis through a time-frequency transforming unit not shown here, and then given to the spectrum amplifying unit 301 as an input signal into the encoding device 300. The spectrum amplifying unit 301 amplifies a spectrum included in every predetermined band with one certain gain. The spectrum quantizing unit 302 quantizes the amplified spectrum with a predetermined transform expression. In the case of AAC method, the quantization is conducted by rounding off frequency spectral data which is expressed in floating points into an integer value. The Huffman coding unit 303 encodes the quantized spectral data in a set of certain pieces thereof according to Huffman coding, and encodes the gain in every predetermined band in the spectrum amplifying unit 301 and the data that specifies the transform expression for the quantization according to Huffman coding, and then transmits the codes of them to the encoded data stream transfer unit 304. The Huffman-coded data stream is transferred from the encoded data stream transfer unit 304 to a decoding device via a transmission channel or a recording medium, and reconstructed as an audio signal on the time axis by the decoding device. The conventional encoding device operates as described above.
In the conventional encoding device 300, a capability for compressing data amount depends on the performance of the Huffman coding unit 303 or the like, so when the encoding is conducted at a high compression rate, that is, with a small amount of data, it is necessary to reduce the gain sufficiently in the spectrum amplifying unit 301 and encode the quantized spectrum stream obtained by the spectrum quantizing unit 302 so as to make it a smaller amount of data in the Huffman coding unit 303. However, if the conventional encoding device 300 structured as above encodes with the smaller amount of data, the frequency bandwidth for reproduced sound and music becomes narrow. So it cannot be denied that the sound and music would be fuzzy for human hearing. As a result, it is impossible to maintain the sound quality. That is a problem.
The present invention is devised in view of the above-mentioned problem, and aims at providing an audio signal encoding device and an audio signal decoding device capable of decoding wide-band frequency spectral data with a small amount of data.